For the last several years, we have witnessed a great increase in the amount of people who either run or jog as a recreational sport. However, concommitant with this increase, is a large number of injuries or conditions which are produced from utilizing athletic shoes which do not properly cushion the user's feet.
As is recognized by those skilled in the art, as well as those millions of recreational runners, good impact absorption and flexibility are two extremely important characteristics desired in athletic shoes. The athletic shoe sole should permit a great deal of flexibility at the point where the foot naturally flexes, while being sufficiently tough to withstand shock, yet soft enough to provide adequate cushioning and comfort. This flexible, yet strong feature is very useful in non-athletic, as well as athletic shoes.
The sole structure of most commercially available today is a tripartite construction including an outer sole, a mid-sole and an inner sole. The outer sole is normally formed of a tough, abrasion-resistant material, since it is the portion of the sole which contacts the ground. The mid-sole is the portion of the shoe between the outer sole and the inner sole and its function is to provide lift for the heel and cushioning for the entire shoe. The inner sole is normally used to join the mid-sole to the shoe structure itself.
Most mid-soles commercially available at the present time utilize a single layer of compressionable rubber-like or similar material. The force needed to initially compress this material is minimal, but as additional force is placed upon the material, a greater amount of force is needed to further compress the material.
U.S. Pat. Nos. 2,968,105 issued to Rizzo; 3,079,707 issued to Hack et al; 3,087,262 issued to Russell; 3,172,217 issued to Colman and PCT Application WO No. 81/01234 show the use of either mid-soles or outer soles which are provided with ribs or ridges which contain at least one portion which is slanted with respect to the normal plane of the shoe. These particular shoe sole designs initially deflect when a force is applied to the sole. Contrary to the compressive force, the force initially used to cause a deflection of the ribs or ridges is rather large, but as additional force is applied, the average amount of force needed to produce additional deflection lessens.
While it has been determined that the use of a material able to deflect when force is applied thereto provides a better cushioning surface than utilizing a single sheet of material for the mid-sole, it has also been determined that a shoe which provides even more cushioning must be developed.